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Old 01-07-2013, 05:13 PM
Telly Telly is offline
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Default 40 backfire

My 78 '40 lets out a healthy backfire when I shut it down. The PO told me to simply engage the clutch in first gear to stop the engine before the "bang". I've been doing this without issue but would rather fix the core problem. The PO did rebuild the carb...can't remember who did the work off the top of my head.

Any ideas where to look first? Thanks. Doug
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Old 01-07-2013, 06:05 PM
spectre6000 spectre6000 is offline
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A bit more info in regards to overall performance and condition would be helpful, but given the info at hand... How's the temp gauge reading?

Backfires happen for a number of reasons. Generally they happen while running though...

The most common cause in my experience is exhaust leaks. Oxygenated air gets in with the hot exhaust gases that still have a little unburnt fuel, and it'll spontaneously ignite causing a backfire. They're pretty easy to track down using only your ears. This requires the engine to run and is not likely your issue.

Another related cause is a poorly tuned carburetor. Running too lean (a lot too lean) can cause the mixture to not be burnable in the cylinder. It goes out the exhaust and ignites when it comes in contact with the hot gases from cylinders that did burn. This will cause a lot of additional issues that will out it pretty easily. Again, must be running, you'd be crawling for lack of power, and it would run pretty badly.

Running too rich can cause backfires in a similar manner, though this usually needs to happen in connection with the aforementioned exhaust leak.

Crazy timing issues can cause intake backfires. These are usually pretty loud and dramatic... Crossed spark plug wires are typically more culpable than anything... It's hard to actually be off that far with your timing and still run. That said, really bad spark plug wire insulation can do weird things if things get crossed. Things arc in weird ways sometimes. Again though, gotta be running.

Finally, your most likely scenario: Hot spot/dieseling. Something is causing your engine to run way too hot and there is enough residual heat after shutdown to ignite fuel (typically in whatever cylinder was on the intake stroke) shortly after shutdown. This will be an intake backfire, so it'll be a bit more dramatic (a loud sharp pop under the hood versus a more subtle pop from the exhaust). This could also manifest itself in the engine continuing to run for a revolution or two after the key has been turned off. Culprits here include timing being way off or running pretty lean. Timing being too far off will cause the whole engine to be pretty hot as the burn happens at such a time that it is in contact with the cylinder bore versus the exhaust system. Running too lean will cause it to get hot in certain places (i.e. spark plug electrodes, high spots between the valves/spark plugs in the head) and these hot spots can effectively make up for the lack of an actual spark.

Suggested action: check your timing first because it's the easiest. Then pull a spark plug and see what it tells you. It's possible that it requires one circuit or another on the carb, so it may be that around town driving doesn't cause you ailment, or that highway driving doesn't cause it.
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Old 01-07-2013, 10:32 PM
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Spectre6000, you are a treasure-trove of information. You need to come hang out with us more often!
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Old 01-08-2013, 07:01 AM
Telly Telly is offline
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The backfire is coming from the exhaust pipe. It only happens after complete engine warm up and driving for 10 plus minutes. I'll check timing and the plugs. Thanks.
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Old 01-08-2013, 10:02 AM
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Quote:
Originally Posted by Telly View Post
The backfire is coming from the exhaust pipe. It only happens after complete engine warm up and driving for 10 plus minutes. I'll check timing and the plugs. Thanks.
Mine has done this a time or two after a long highway drive and then engine shut down. I probably need to adjust timing.
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Old 01-08-2013, 12:57 PM
spectre6000 spectre6000 is offline
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Quote:
Originally Posted by nattybumppo View Post
Spectre6000, you are a treasure-trove of information. You need to come hang out with us more often!
Thanks! I fully intend to. I'm a few received shipments from being up and running, and I hope to be fairly active. Our friends back in Dallas were all car club people, and our social life was more or less based on it, so we're eager to get back in the game.
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Old 01-08-2013, 02:57 PM
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My old 76 Fj40 used to do that back in CA. It didn't do it every time though. Kind of humorous in a parking garage. It was due to a burnt valve.
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Old 01-09-2013, 05:23 PM
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Rzeppa Rzeppa is offline
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My 60 has backfired like crazy when going downhill with throttle closed ever since engine rebuild. I wasn't going to bother to post until I had checked valve adjustment (they always get closer with a new cam and lifters) and replaced the muffler with all the holes in it.
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Old 01-09-2013, 06:58 PM
spectre6000 spectre6000 is offline
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The closed throttle means there is very little fuel coming from the carb, yet the engine is still turning due to the movement of the car through the transmission. The mixture in the cylinders is too lean to consistently burn, so some of it is making it to the exhaust where it is meeting the hot gasses from cylinders that did burn and igniting in your exhaust. The holes in the exhaust obviously aren't helping and may make any tuning attempts moot, but you may find turning your idle mixture screw out a quarter to half turn will rectify this to some extent.
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Old 01-09-2013, 08:03 PM
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Quote:
Originally Posted by spectre6000 View Post
The closed throttle means there is very little fuel coming from the carb, yet the engine is still turning due to the movement of the car through the transmission. The mixture in the cylinders is too lean to consistently burn, so some of it is making it to the exhaust where it is meeting the hot gasses from cylinders that did burn and igniting in your exhaust. The holes in the exhaust obviously aren't helping and may make any tuning attempts moot, but you may find turning your idle mixture screw out a quarter to half turn will rectify this to some extent.
I know all this; however the carb is perfect, it is a Jim C. rebuild and was working perfectly prior to the motor rebuild. The mixture upon negative acceleration is not too lean, it is just right. The idle mixture is also just right, and the jets are perfect for my elevation.

Once I check the valves (and adjust if necessary) and replace the exhaust, I will post up if I need any help. That's why I haven't posted - I know what to do and am not stumped, right now I am working on a rear bumper project for that rig that I am halfway through.

Hugh's guys at Safari LTD did the rebuild, and while they are pretty good, a fully smogged 60 has a buttload (tech term) of emissions subsystems that could have been reconnected incorrectly, and there are lots of places for an incorrectly connected emissions subsystem to cause backfiring as well. I have plenty of (really time consuming) experience in troubleshooting each subsystem and have been putting that off until after I check the valves and replace the exhaust.
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